1. Field of the Invention
The present invention relates to an image forming apparatus which transfers toner images of respective color components onto a printing medium to obtain an image using an electrophotographic method or electrostatic printing method, and a control method for the image forming apparatus.
2. Description of the Related Art
Conventionally, an apparatus shown in FIG. 25 is used as a color image forming apparatus. A developing means comprises a magenta toner developing unit 3M, cyan toner developing unit 3C, yellow toner developing unit 3Y, and black toner developing unit 3K. A rotary developing unit 3 is supported to be rotatable by a rotation support device (not shown). The above-described color toner developing units develop the images using corresponding color toners while opposing a photosensitive drum 4 in turn.
With this structure of developing means, the photosensitive drum 4 is rotationally driven at a predetermined angular velocity, so the surface of the photosensitive drum 4 is uniformly charged by a charger 8. ON/OFF of a laser beam is controlled in correspondence with image data of the first color (e.g., magenta) to form an electrostatic latent image of the first color on the photosensitive drum 4. The obtained image is developed and visualized by the magenta toner developing unit 3M of the first color. While this visualized first toner image is brought into press contact with the photosensitive drum 4 with a predetermined force, it is transferred onto a printing medium 6 held on the surface of a rotationally driven transfer drum 5. The above-described transfer process is similarly repeated for the other toners (yellow, cyan, and black). Every time this process is repeated, toner images formed with respective color toners contained in the corresponding developing units are in turn transferred and superimposed on the printing medium 6 carried on the transfer drum 5, thus forming a color image. The printing medium 6 is fixed by a fixing unit 7 and discharged outside from the image forming apparatus.
At this time, the above-described color image forming apparatus cannot obtain a full-color image until the transfer drum 5 rotates several times. Therefore, a full-color image is formed at a slower speed than in a monochrome image which can be obtained by rotating a transfer drum once.
To solve this problem, it is common practice to transfer a plurality of images onto a transfer drum at once so as to simultaneously obtain a plurality of full-color images by driving the transfer drum in one sequence, thus speeding up full-color image formation. For example, as shown in FIG. 26, two printing media 6A and 6B are simultaneously carried at corresponding reference positions PA and PB of the peripheral surface of the transfer drum 5 (to be referred to as two-page printing hereinafter). Therefore, the transfer drum 5 can simultaneously transfer two full-color images onto the two printing media 6A and 6B per rotation.
In recent years, it is necessary to increase the speed of a monochrome output so as to cope with the demands of office use.
Under these circumstances, the wait time (warm-up time) from when the image forming apparatus returns to a normal power supply mode (to be referred to as activation hereinafter) after power ON or from a low-power mode until the output actually becomes ready (standby) causes the users great inconvenience. Temperature control and image adjustment of a fixing unit occupy most of the activation time after power ON.
Conventionally, temperature control of a fixing unit is very important. High-temperature stable temperature control is therefore demanded to develop colors and fix toners by melting the toners and mixing the colors. In particular, assume that the power supply is turned on while the fixing unit has a low temperature upon having been left unused. Upon such activation, problems for minimizing the time required to raise the temperature of a fixing roller and for controlling the temperature of the entire region of the fixing roller without any variation exist. As a measure against this problem, there has conventionally been proposed a technique for using a fixing roller made of a high-thermal-conductivity material, or a technique for forming a thin surface layer of a fixing roller. Another approach uses toner which can readily melt without any variation even at a low temperature.
Furthermore, along with a recent increase in full-color output, assurance of the density stability and tone stability of an output image is being demanded. To meet this demand, the following techniques have been known as image control methods for an image forming apparatus.
Japanese Patent Laid-Open No. 2001-75318 discloses the following technique. An image forming apparatus is activated and performs a warm-up operation. After completion of that operation, the apparatus forms a solid image pattern and halftone image pattern, and reads the densities of these patterns. On the basis of the read density values, the apparatus executes maximum density control and tone control which change the operation of a circuit such as a γ correction circuit which determines the image formation condition, thus stabilizing the quality of an image to be formed.
Japanese Patent Laid-Open No. 10-240082 discloses the following technique. Even when the tone characteristic of a specific pattern changes upon a variation in environmental conditions, an image forming apparatus forms and reads that pattern again. The apparatus feeds back the result again to a circuit such as a γ correction circuit which determines the image formation condition, thus stabilizing the image quality in accordance with the variation amount of that environmental condition.
In Japanese Patent Laid-Open No. 10-240082, when the image forming apparatus is used over a long period of time, the density obtained by reading the pattern on an image carrier may not coincide with the density of an image actually printed out. To solve this problem, there has further been known a technique for forming a specific pattern on a printing medium to correct the image formation condition depending on its density value. There has still further been known the following technique. An image forming apparatus forms a specific pattern in a non-image region during an image formation operation and reads the density of that pattern. On the basis of the read density value, the apparatus changes the operation of a circuit such as a γ correction circuit which determines the image formation condition for every image formation operation, thus accurately correcting the image characteristic which varies every moment.
However, in a recent color image forming apparatus, the user who wants to output a monochrome image or the user who wants to output a business document with little concern for tonality immediately after activation must wait for a standby state until the image adjustment is finished. Therefore, activation as fast possible is demanded.
To meet this demand, in Japanese Patent Laid-Open No. 2002-44309, an image forming apparatus recognizes at the time of activation whether the job of interest is a monochrome job or color job. The apparatus omits image adjustment if that job is a monochrome job, and executes image adjustment if that job is a color job. Japanese Patent Laid-Open No. 2002-44309 can assure the density and tone for a color job after activation. However, the activation time for a color job remains the same as that in the prior arts.
That is, a color image forming apparatus controls the temperature of a fixing unit and executes an image adjustment operation after completion of a warm-up operation, so the wait time does not shorten. To shorten the wait time, when both the operations are simultaneously executed, a large amount of power is required. This measure falls behind recent moves for energy savings.